a. Field of the Invention
This invention relates generally to single particle absorption spectrometry and more particularly to spectrometry in which a photoacoustic technique is used to measure the absorption spectra for a single aerosol particle.
b. Description of the Prior Art
The disclosures of all such publications referenced herein, in their entireties, are hereby expressly incorporated by reference in this application as if fully set forth, for purposes of indicating the background of the invention and illustrating the state of the art.
Recent experiences in the Gulf War and with the Sarin poisonous gas attack in a Japanese subway have demonstrated the susceptibility of both military and civilian personnel to chemical/biological aerosol attacks and the need to develop some type of early warning system. Current methods for real time biological aerosol detection attempt to exploit the relatively weak fluorescence phenomena inherent in all living materials, G. W. Faris, R. A. Copeland, xe2x80x9cSpectrally Resolved Absolute Fluorescence Cross Section of B. Globigii and B. Cercusxe2x80x9d, Stanford Research Institute, Technical Report 2913, (1992). Unfortunately, measured fluorescent spectra are often quite broad and featureless, making species discrimination and/or identification nearly impossible. It has become apparent that additional criteria must be considered, i.e., electromagnet (EM) absorption, if effective identification schemes are to be developed. In addition, researchers have pointed out that for any biological warfare (BW) early warning detection method to be truly effective it must be able to detect biological particulate concentrations on the order of 10 bio-particles per liter of air. Currently, all known detection schemes require bio-particle concentrations several orders of magnitude greater than this minimum detection limit.
In order to solve this complex and detailed problem, detailed absorption spectra derived directly from in situ bioaerosols, preferably a single particle, is badly needed. Prior scientific studies involving EM absorption by aerosols have used a conventional photoacoustic approach in which an ensemble or distribution of aerosol particles were necessary to conduct the measurement, C. W. Bruce, xe2x80x9cDevelopment of Flow Through Spectrophones for CW and Pulsed Radiation Sourcesxe2x80x9d, ECOM Tech. Report No. 5802, p. 1-57, (1976); C. W. Bruce, K. P. Gurton, T. F. Stromberg, xe2x80x9cTrans-Spectral Absorption and Scattering of Electromagnetic Radiation by Diesel Soot, xe2x80x9d Applied Optics, 30, No. 12, pp. 1537-1546, (1991); and K. P. Gurton, C. W. Bruce, xe2x80x9cMid-infrared Optical Properties of Petroleum Oil Aerosols,xe2x80x9d Army Research Laboratory Technical Report, No. APL-TR-255, White Sands Missile Range, NM, pp.1-43, (1994). Trying to apply this conventional photoacoustic approach as a early warning detection method has two major drawbacks. First, measurements conducted on an ensemble distribution of bioaerosols severely mask detailed spectral content due to averaging effects over both size and orientation of the particles, thus rendering the measured absorption spectra as featureless as fluorescence, A. V. Jelinek, C. W. Bruce, xe2x80x9cExtinction Spectra, of High Conductivity Fibrous Aerosolsxe2x80x9d, J Appl. Physics, 78, 2675, (1995). Second, very high particle concentrations are necessary to achieve good signal-to-noise and operation at the 10 particle per liter criteria would not be feasible.
Because this apparatus/technique can detect absorption spectra from a single aerosol particle, it is uniquely suited for detection at extremely low aerosol densities, i.e., at or below the 10 particle per liter criteria.
Thus, to reiterate, to date there are no rapid, reliable, effective means to detect low concentrations of harmful bioaerosols, that could be used to warn populations at risk in sufficient time to take evasive measures. We believe the device proposed here could satisfy this need.
It is an object of this invention to develop a method and a device capable of:
1) producing the free-space levitation of a single aerosol type particle (via an electrodynamic particle trap) to permit detailed analysis and study of the absorption characteristics without the detrimental effects of ensemble averaging inherent using current techniques;
2) identification and discrimination of hazardous bioaerosols to enable rapid detection of and early warning against a variety of hazardous aerosols;
3) to obtain nonintrusive measurement of absorption spectra of an individual particle in situ, thus allowing for detection at very low aerosol densities.
These and other objects are satisfied, at least in part, by a single particle photoacoustic absorption spectrometer including a particle trap for suspending a charged particle in free-space in a non-intrusive manner; a source of a electromagnetic energy modulated at a convenient acoustic frequency (e.g., a tunable laser chopped at 1 kHz), for rapid heating and cooling of the particle at the same acoustic frequency that results in a compressional/sound wave whose intensity is directly proportional to the EM absorption by the particle at the laser frequency of interest; a very sensitive, highly directional microphone (e.g., miniature electret microphone) positioned to detect the acoustic signal emitted by the suspended particle and; a resonant cylindrical cavity to amplify the acoustic wave. Thus the present invention provides an efficient, simple and effective means of suspending a single aerosol type particle stationary in free-space so that thermal changes in the particle (induced by optical absorption of the irradiating source) can be measured via the resultant acoustic signal.
Still other objects, features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. As will be realized, the invention is capable of other different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not restrictive.